In the oxygen refining of pig iron it is necessary to monitor how full the refining crucible is. In particular it is useful to know the thickness of the slag layer that forms on top of the melt during the refining operation.
It has been found that a convenient method of determining this slag thickness is by monitoring the noise produced by the oxygen jet directed at the melt. To this end a microphone is mounted near the top of the crucible and detected sound is transformed into an electrical signal whose intensity readily can be measured. In order to achieve the most advantageous ratio between useful and parasitic noises and in order to minimize distortions it is necessary to have the microphone as close as possible to the mouth of the crucible.
It is therefore necessary to provide a cooling system for the microphone. This is typically done by means of water-filled coolant tubes surrounding the microphone. A considerable disadvantage of this system is that the extreme heat frequently causes the water in the coolant to boil, thereby greatly increasing the extraneous noise level. Furthermore, the microphone invariably has a very limited service life due to its location close to the hot refining crucible.